Study of regional temperature and thermohydrological effects of an underground repository for nuclear wastes in hard rock

Abstract
Heat released by the radioactive decay of nuclear wastes in an underground repository causes a long-term thermal disturbance in the surrounding rock mass. The nature of this disturbance for a planar repository 3000 m in diameter at a depth of 500 m below surface is investigated for various waste forms. The effects of changes in the density and viscosity of groundwater caused by the temperature changes on the flow through a simple model of a vertical fracture connected to a horizontal fracture in a rock mass are evaluated. It is concluded that different waste forms and time periods before burial have significant effects on the thermal disturbance and that buoyant groundwater flow is a function of both the vertical and horizontal fracture transmissivities, as well as the changes in temperature. Loaded initially with a power density of 10 W/m/sup 2/ of spent fuel assemblies 10 years after discharge from a reactor, the maximum increase in temperature of the repository in granite is about 50/sup 0/C and the epicentral thermal gradient about 70/sup 0/C/km. 30 figures, 4 tables.

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